Effect of Different Numbers of Spatial Confinement Walls on Laser-Induced Cu Plasma Spectra

Objective Laser-induced breakdown spectroscopy is a powerful spectroscopy technique for the analysis of various materials. As we all know, laser pulse ablates the sample and generates plasma, and the shock wave generated by the plasma propagates at supersonic speed. When the shock wave encounters the wall of confinement cavity, it will be reflected. The reflected shock wave compresses the plasma, increasing the collision rate of particles in the plasma. It will increase the number of atoms in the high-energy state and enhance the plasma's spectral intensity. As a result, many researchers have conducted extensive research on spatial confinement. However, no research group has yet investigated the effect of the number of spatial confinement walls on the spectra of laser-induced plasmas. For this reason, it is necessary to compare and study the plasma spectral emission characteristics for the different spatial confinement walls. Methods We focused nanosecond laser on the surface of the Cu target to generate plasmas, and analysed the spectra of the generated plasmas. To avoid laser irradiation at the same position on the target surface, the Cu target was placed on a 3D translation stage and moved along with it. The focusing lens collected the plasma emission signal, which was then transmitted via optical fibre to a spectrometer equipped with an ICCD. To ensure time synchronisation between the laser and spectral signals, the output laser triggered the photodiode. The cavities with different spatial confinement walls (2, 3, 4, and cylindrical walls) were machined using aluminium alloy. Each spectrum was an average of 20 laser shots, and the whole experiment processes were carried out in the air. Results First, the spectra for different numbers of confinement walls at a delay time of 12.5 mu s were compared. The results show that the number of confinement walls has a significant enhancement effect on the intensity of the three Cu (I) spectral lines at a delay time of 12.5 mu s. Second, the time-resolved spectral intensity of Cu (I) at 521.82 nm was measured for various numbers of confinement walls. Within the acquisition delay range of 9-22 mu s, the intensities of the Cu (I) for the number of confinement walls of 2, 3, 4, and cylindrical wall are stronger than those for the number of confinement wall of 0 (Fig. 3). The laser irradiates on the Cu target surface at the centre of the confinement cavity to generate plasma, and the shock wave generated by the Cu plasma rapidly expands (the expansion speed of the shock wave is much faster than the diffusion speed of the plasma). During the plasma expansion process, the shock wave will be reflected by the cavity wall, and the reflected shock wave will compress the Cu plasma to a smaller volume. Because energy cannot be rapidly diffused in plasma, it is absorbed by low-level atoms and transitions to the high-level, causing the intensity of the Cu (I) line to increase within the acquisition delay range of 9-22 mu s. Third, the comparison of best enhancement factors of Cu (I) at 521.82 nm for the different numbers of confinement walls at a delay time of 12.5 mu s was analyzed. It is found that when the confinement cavity is the cylindrical wall, the best enhancement factor of Cu (I) at 521.82 nm is the highest (Fig. 4). At this point, the shock wave will be reflected by the cylindrical wall, resulting in increased shock wave energy and a greater degree of coupling between the shock wave and plasma plume (Fig. 5). Fourth, the time-resolved SBR of Cu (I) at 521.82 nm for various confinement walls was measured. It is found that when the confinement cavity is the cylindrical wall, Cu (I) at 521.82 nm has the maximum SBR (Fig. 6). Finally, the time-resolved plasma temperature for the different numbers of confinement walls was calculated by the Boltzmann diagram method. Plasma temperature changes are similar to changes in spectral intensity and SBR, and the electron temperature with a confinement cavity of a cylindrical wall is the highest (Fig. 7). As previously stated, when the confinement cavity is the cylindrical wall, the spectral intensity, SBR, and electron temperature are the highest; that is to say, the spatial confinement effect is the best at this time. Conclusions In this paper, the influence of the number of spatial confinement walls on laser-induced Cu plasma spectra was studied in an atmospheric environment. The experiment discovered that the spectral intensity, SBR, and electron temperature of Cu plasma increased with increasing spatial confinement walls; when the confinement cavity was the cylindrical wall, the spectral intensity, SBR, and electron temperature of the plasma were the highest. The spatial confinement effect resulted from the shock wave reflected from the confinement cavity wall compressing the plasma plume. As the number of confinement walls increased, the energy of the shock wave used to confine the plasma continued to grow, and the coupling degree of the shock wave and plasma plume was also increasing, resulting in the continuous enhancement of the compression effect of the plasma. In conclusion it can be seen that a sufficient number of spatial confinement walls can effectively increase the spectral intensity, SBR, and electron temperature, there by improving spectral signal and sensitivity.